1. Field of the Invention
This invention relates to a method and apparatus for improved particulate mixing in a vessel rotating about its central axis and rocking in a direction perpendicular the central axis.
2. Description of the Related Art
Dry particulate mixing is a basic operation frequently used in a wide variety of industrial applications, such as food, agricultural products, cosmetics, coal, cement, pharmaceuticals, chemicals, plastics, and ceramics. Conventionally, the success of solids mixing operations has been evaluated in terms of ultimate product quality. Inadequate mixing during the production sequence can result in rejection of the finished product due to poor quality. If mixing insufficiencies can be identified and avoided during the manufacturing process, fewer batches would be rejected, thus reducing both waste and manufacturing costs.
One solution describes identifying the particular mixture with a solidification technique to preserve the mixture structure in an undisturbed state. As described by Shinnar et al. in "A Test Of Randomness For Solid-Solid Mixtures", Chemical Engineering Science, Vol. 15, pp. 220-229 (1961), samples are solidified with molten wax and surface samples are produced by slicing the solidified sample with a microtome. Thereafter, observation and analysis of flow patterns can be determined from the solidified structure.
In another approach, binary mixtures of coarse ceramic powders were impregnated with an epoxy resin, see Lin et al. "Assessment of Uniformity of Composition Processing" in Processing of Advanced Ceramics, pp. 1-18, Soc. Esp. Cerma. Vidir. Arganda del Rey, Madrid, Spain, 1986. After the resin has set, the resin is cut into disc-shaped sections. The sections were subsequently ground, polished and analyzed. This approach has been used on small samples and as described enables assessment of mixing by examination of the local mixture composition.
Numerous steadily rotating mixers have been described, such as rotary kilns, dryers, ball mills and spray coaters. U.S. Pat. No. 4,491,415 describes a pear shaped rotary drum mixing device in which the drum is rotated around a central axis. The drum is supported at an angle of delineation of about 35.degree.. A plurality of radial fins within the drum lift the contents during rotation thereof.
A study of the mixing in the radial phase of a steadily rotating horizontal mixer described that mixing depends on the type and level of loading of the mixture in the horizontal cylinder; see K. W. Carey-Maccauley and M. B. Doruld, The Mixing Of Solids In Tumbling Mixtures, Chemical Engineering Science 1964, Vol. 19, pages 191 & 199. The results indicated a steep increase in mixing time as filling of the horizontal cylinders reached the half way mark.
Axially rotating cylinders have the shortcoming that the steady rotation often results in slow mixing and non-uniform distribution of components in the mixer, as described in D. S. Cahn et al, Nature, 209 (1966) 494. Horizontal and cylindrical kiln particles tend to move along recirculating flow patterns and can become trapped in dead regions. The trapped particles are slowly blended with the other particles in the system.
The mixing performance of a rotary drum with simultaneous axial rocking motion was described by M. Alonso et al, Influence of Rocking Motion On The Mixing of Powders, Powder Technology, 59 (1989) pages 65-67. Mixing in horizontal rotating cylinders was improved when the mixer was rocked back and forth in the axial direction. Relatively high rocking speeds provided high mixing rates regardless of the rotation speed. The temporal variation of the state of mixing was continuously measured by an optical method.
Work in fluid mixing described in M. Liu, F. J. Muzzio and R. L. Pesking, Quantification of Mixing in Aperiodic Chaotic Flows, Chaos, Solits, Fractals, to appear, 4(6):869-893, 1994; D. J. Lamberto, F. J. Muzzio and P. D. Swanson, Using Time-Dependent RPM To Enhance Mixing In Stirred Vessels, Chemical Engineering Science, Vol. 51, No. 5, pp. 733-741, 1995 has demonstrated that flow perturbations can be used to enhance mixing performance in industrial equipment. Typical process equipment used in industrial applications used time-periodic or spatially-periodic flows to mix materials (examples of the former are stirred tanks with steadily rotating impellers and also tumbling blenders rotating at constant speed; examples of the later are static mixers and also extruders). In many cases, such periodic flows produce a substantial amount of recirculation, leading to the creation of segregated flow regions and often resulting in incomplete mixing. Liu and Muzzio (1994) used theory and computations to show that the introduction of flow perturbations (and, in particular, perturbations that destroy all flow periodicity) are an effective and robust approach for destroying such segregated patterns, resulting in large mixing enhancements. Lamberto et al. (1995) demonstrated that this method could be used to enhance mixing in liquid mixers of interest to industry. Wightman et al., A Quantitative Image Analysis Method For Characterizing Mixtures of Granular Materials, Powder Technology 1995 also demonstrated that the perturbation method is effective in enhancing powder mixing.
It is desirable to provide a system for improved mixing and identifying the mixing characteristics during production.